This invention relates to copiers in general, and in particular to an electro-photographic copier.
Copying apparatus of this type operates on the xerographic principle, i.e. an electrostatic charge is formed on a carrier surface and is then partially negated, leaving on the carrier surface a patterned charge corresponding to the image of an original (or of an original copy) to be reproduced. Powdered xerographic material is then applied in a developing station to the carrier surface, whereas it adheres at those portions of the surface which still carry the charge.
The powdered xerographic material is stored at the developing station in a receptacle and cascaded over the carrier surface -- usually the circumferential surface of a xerographic drum -- by a small bucket conveyor or another type of conveying device. Since the xerographic material must be replenished from time to time, which can be done most conveniently outside the relatively crowded confines of the machine housing, it is known to arrange the components of the developing station, including the receptacle and conveyor, in form of a developing unit that can be pulled out of the machine for replenishment of the xerographic material and be pushed back into the machine to operative position adjacent the xerographic carrier. Such an arrangement has the additional advantage that the components of the unit are readily accessible for servicing when the unit is pulled out of the machine. When the unit is pushed into the machine into its operative position, it is latched in place with a suitable arrangement.
The conveyor, whether or not it be of the bucket type, is constantly loaded with a quantity of xerographic material, which is being advanced to the point at which it becomes discharged from the conveyor to cascade over the xerographic carrier. Thus, in the case of a bucket conveyor, there will always be a full bucket located adjacent the discharge point and ready to discharge its contents. Unlatching the developing unit and withdrawing it outwardly from its operative position, tends to cause these contents to become discharged at a time when this is not desired, i.e. when due to the outward movement of the unit the material will not be discharged onto the xerographic carrier but generally into the interior of the machine.
It is self-evident that this is intolerable because it would very rapidly dirty the interior of the machine. For this reason the prior art has proposed to provide a switch which is automatically operated when the developer unit is unlatched in preparation for withdrawal. The switch, in turn, initiates a brief reversal of a tandem motor which serves to drive the conveyor. In consequence, any xerographic material which approaches the discharge point on the conveyor, is moved back from the discharge point by a distance sufficient to prevent it from becoming accidentally and unintentionally spilled into the machine.
This principle is very sound; however, the structural solutions for carrying the principle into effect are rather complicated and expensive. They require the use of electrical and electronic components, such as the aforementioned tandem motor, relays, a timer or timing stage, and the like.
This is clearly undesirable, not only because the increased manufacturing expenses reflect unfavorably on the salability of the machine, but also because the relative complexity of these constructions tends to make them susceptible to malfunction which in turn undesirably increases maintenance expenses.